1. Field of the Invention
This invention relates to a valve timing control system for an internal combustion engine, and more particularly to a valve timing control system for an internal combustion engine for changing the valve timing according to the operating condition of the engine.
2. Description of the Prior Art
Generally it is preferred from the viewpoint of engine performance that the valve timing of the intake valve and/or the exhaust valve be changed according to the operating condition of the engine. For example, during operation of the engine under light load, the valve overlap, i.e., the time that both the exhaust valve and the intake valve are open, is preferred to be short to reduce the amount of residual exhaust gas, whereby stability of combustion in the engine can be ensured. Further, during operation of the engine at a low speed under heavy load, backflow of intake gas can be prevented and the volumetric efficiency can be improved by reducing the valve overlap. On the other hand, in order to improve the volumetric efficiency during heavy load high speed operation of the engine, the valve overlap is preferred to be long.
Thus there have been proposed various valve timing changing mechanisms for an internal combustion engine. For example, there is disclosed in Japanese Patent Publication No. 52(1977)-35819 a valve timing changing mechanism in which a planet gear associated with a centrifugal governor is interposed between the output shaft and the camshaft of the engine to change the relative position of the camshaft to the output shaft. In another valve timing changing mechanism, a three-dimensional camshaft is used as the camshaft and the three-dimensional camshaft is slid to change the valve timing.
However, the conventional mechanisms are disadvantageous in the they are significantly complicated in structure, are apt to produce loud noise, respond poorly and have low reliability.